1. Field of the Invention
The present invention relates to a variable reluctance type resolver, and an electrically powered motor having such a resolver and a power steering apparatus.
2. Description of the Related Art
Conventionally, a hydraulic power steering apparatus is used as a means to assist steering of an automobile or the like in which a pump generates, by an engine output, a hydraulic pressure so as to assist steering. Also, an electrically powered steering apparatus (hereinafter, referred to as an “EPS”) is used as a means to assist steering of an automobile or the like in which a motor inside a car battery of the automobile rotates the motor. The EPS uses no engine output and is, compared with the hydraulic power steering apparatus, a more efficient system.
The EPS requires an angle detection apparatus which is operable to detect with high accuracy a rotational angle of the motor in order to achieve a high accuracy control of the system. Automobiles are required to operate accurately and for a long period of time in various types of environments. Therefore, the EPS which is a component in an automobile is required to be reliable in the aforementioned various environments.
The resolver of the variable reluctance type which achieves high accuracy and which is, compared with an optical encoder or a magnetic encoder, high in environmental resistance is used as the angle detection apparatus for the EPS. In such resolver, however, harmonic components in the induced voltage generated in the output coil may have an effect on the accuracy of the detection, and a configuration of the rotor of the resolver may have an effect on the harmonic components. Therefore, there are various technologies available for determining the configuration of the rotor in order to increase the accuracy of the detection of the resolver.
For example, Japanese Laid Open Patent Publication No. 2698013 discloses a technology for reducing detection error caused by harmonic components included in induction voltage of an output coil. Also, Japanese Laid Open Patent Publication 3200405 discloses a technology for reducing detection error caused by magnetic flux leaked from a rotor. Also, Japanese Laid Open Patent Publication No. 2005-49183 discloses a technology for approximating a gap permeance between a rotor and a stator to a sine wave pattern.